Biographer's material, what's known of it? (3,5,2,4)
I believe the answer is:
the facts of life
Here is my best explanation:
'biographer's material what's known' is the definition.
I can't tell whether this defines the answer.
'of it?' is the wordplay.
I cannot really see how this works, but
'of' could be 'o' and 'o' is found in the answer.
'it?' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
This explanation may well be incorrect...